This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. Fluorescence resonance energy transfer (FRET) between two fluorophores is a function of the distance between the fluorophores. This property is used experimentally to create sensors of biological activity and monitor changes in protein structure. FRET is also applied in living cells to measure the proximity of proteins within a complex that is tagged with a pair of fluorescent proteins. However this last application is hampered by the lack of a statistically robust methodology. FRETSCAL is an integrated collection of Matlab scripts for FRET analysis that provides the statistically robust methodology. Fluorescence resonance energy transfer (FRET) between two fluorophores is a function of the distance between the fluorophores. This property is used experimentally to create sensors of biological activity and monitor changes in protein structure. FRET is also applied in living cells to measure the proximity of proteins within a complex that is tagged with a pair of fluorescent proteins. However this last application is hampered by the lack of a statistically robust methodology. FRETSCAL is an integrated collection of Matlab scripts for FRET analysis that provides the statistically robust methodology.